


Blue-Eyed Monster

by ForgiveMeForIHaveSinned (orphan_account)



Category: Undertale (Video Game)
Genre: Aggressive Sans, Everything is completely consensual though bc that's how I roll, F/M, Other, Polyamory, Reader Is Not Frisk, Sensitive bones, Soul Sex, The fontcest is not explicit, mentions of ecto-junk but it doesn't come into play
Language: English
Status: Completed
Published: 2015-12-18
Updated: 2015-12-18
Packaged: 2018-05-07 09:47:26
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 2,775
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/5452268
Author URL: https://archiveofourown.org/users/orphan_account/pseuds/ForgiveMeForIHaveSinned
Summary: <blockquote class="userstuff">
              <p>You've been dating Papyrus for a while now, and you've recently taken your relationship to a more physically intimate level. You and Papyrus are very happy about this. Sans...isn't, and you think you might know why. </p><p>(Post-pacifist ending I guess, gender-neutral reader, no gendered pronouns.)</p>
            </blockquote>





	Blue-Eyed Monster

**Author's Note:**

> OH MY GOD, I DON'T EVEN SHIP FONTCEST but this idea dropped fully-formed into my head and it would not leave no matter WHAT I did - AND I TRIED A LOT OF THINGS. No hate for those of you who ship it, obviously. This has just been a very confusing week. I hope you enjoy it!

When you arrive at the skeleton brothers' house, Papyrus isn't home. You figured he'd still be out at work - the Captain of the Royal Guard had an excellent work ethic, after all - so you took your groceries into the kitchen and started putting your groceries in the fridge and pantry. Maybe tonight you could finally convince Papyrus to try penne. It had taken some persuasion to get him to agree to angel hair and linguine, but you figured slow and steady was the way to go with Papyrus.

You find yourself blushing as you close the fridge. How did you manage to make an innuendo inside your own head? It was probably because the memory was still fresh in your mind - you and Papyrus had only become physically intimate last night. ...no, that's not quite right. You'd only become _sexually_ intimate last night. You'd been physically close to him for a good few months now, snuggled up on the sofa watching TV or cuddling in his race car bed. You'll never forget the first time he lay his head on your chest, listening for your heartbeat through the layers of your clothes.

Despite yourself, you sigh a little as you fold up your grocery bags and put them in the drawer. Your fingertips still tingled a little when you thought of the rough edges of his bones. It had been nothing and everything like you'd expected - Papyrus had been so curious about your body, about the way your touch had affected his. It hadn't felt weird, though, despite how different your bodies were. Seeing the wonder in his eyes as he explored you had been so...wonderful.

"Hey."

You jump; you hadn't noticed Sans come downstairs, but there he is, lounging on the couch like he'd been there all afternoon.

"Hey," you shut the bag drawer and quickly check the cabinets for dogs (still none, but it's gotten to be a habit). "I thought you were gonna be with Alphys all day."

"It was a short day," Sans replies, and as you shuffle into the living room and flop down on the couch beside him, he looks tense.

You don't really know what to say to that, so you sort of just lean back and look at the ceiling. This isn't unusual; doing nothing was one of Sans's favorite things to do, and he didn't mind company. This time, though, the silence wasn't nearly as comfortable as you were used to. 

"Papyrus slept in this morning," Sans said, and even though his tone was casual, you couldn't bring yourself to look at him.

"I...don't know if I'd call it 'sleeping in'," you shrug, feeling a chill work its way down your spine. "I mean, he woke up early enough to get to work on time."

When your neck is finally too uncomfortable and you lower your head, Sans is standing right in front of you.

"Papyrus has never woken up that late," he leans forward, the glow of his eye unusually bright.

"Uh," you wonder if you should be afraid. (Sans has _never_ given you cause to be afraid of him, not once. What is this?) "I mean, he was probably just tired."

"You'd know, wouldn't you?" Sans hisses, his words nearly silent in your ears. "y o u  d i r t y  b r o t h e r  f u c k e r."

"Where the hell did this come from?" You gasp a little. "What happened to the whole 'Papyrus is old enough to make his own decisions' speech you gave me when we started dating?"

"That was _different_ ," Sans's hands come up and hit the couch cushions on either side of your neck. "Papyrus didn't know anything about that sort of thing until you came along."

"I..." your voice leaves you for a moment. "Are you worried he wasn't ready for it? I'd never push him into something he didn't want, and you know it."

Sans doesn't reply, but you hear him try to say something at least twice. Pieces of memory from the night before start to come together, forming a bigger picture like stained glass.

"I can't tell if you're being protective," you whisper, "...or jealous."

"What?" Sans's eye socket narrows, inches from your face. "You think I'm upset because you chose him over me?"

"No."

For once, it seems like you've managed to surprise him. He drops his hands and stares at you for a moment, eye sockets dark, and then his whole face burns with anger. His eye lights up again so quickly, it his you like the flash bulb of a camera. Spots dance in your vision, and you try to shake them off. 

"I'm not mad," you shake your hands in front of you, trying to keep him from blowing a fuse and denying everything. "I'm really not."

"What the hell do _you_ have to be mad about?" Sans lunges forward again, pinning his arms to the sofa behind you again. His forehead nearly touches yours, and you can feel his legs pressing against your knees. With your eyes finally adjusted, you look up at him with what you hope is a reassuring smile. 

"You're not as quiet as you think you are."

Sans just stares at you for a second, and then finally his forehead _does_ hit yours rather hard.

" _Ah_ ," you turn out of the way and Sans sort of collapses on top of you, so you're both lying sort of sideways on the couch. His face is flushed with blue, and this time you can tell it's not out of anger. He's staring blankly at the air in front of him, looking so...defeated. You've never seen him like this before, and you don't like it.

"I'm sorry," he says, turning away from you.

"Apology accepted," you rub his shoulders gently. "It's okay."

"You're wrong, you know," he murmurs softly.

"What do you mean?"

"I _was_ mad you picked him over me," he leans into your shoulder, and instinctively your wrap your arms around him. "But you were also right."

"You were also mad he picked me over you?" you whisper.

Sans just nods, and he looks so unbelievably sad, you can't stand it. You kiss the top of his skull gently.

"Do you want to know a secret?" you ask.

He looks up at you, a little confused. "...sure."

"I don't think he did."

"What does that mean?" Sans sits up a little. "You've been dating him for -"

"I mean he wouldn't stop talking about you," you interrupt him before he can argue. Thin though the walls may have been, you don't think Sans heard any distinct words. "Even when my hands were on him."

Sans's blush deepens, and a faint blue glow comes up from under the collar of his shirt. You can't believe you recognize it, but after last night it's unmistakable. Without really thinking, you reach towards it.

"Oh!" Sans jumps. "That's uh...I mean-"

"Sorry," you drop your hand. "I don't know what I was thinking. I mean...unless you want to?"

"No," Sans shakes his head, "we shouldn't do that to Papyrus."

"Um," you look away, "actually - well, I don't know if he meant this _exactly_ but he did say -"

"What did he say?" Sans clambers back down, until he's lying against your chest again. "What did he say about me?"

The look in his eyes is so desperate, and it's a hell of a lot more exciting than you bargained for. You feel your body start to heat up, particularly where Sans's knee is resting between your legs.

"He kept asking if..." your mouth suddenly feels very dry, remembering the sweet excitement in Papyrus's eyes. "He kept saying things like, 'Does my brother know about this, human? Can you make him feel good, too?'"

His words sound different in your voice, but you remember them exactly. At the time, you felt a little guilty about how much it turned you on, but that's changing very quickly.

"No way," Sans gasps, "you are making that up."

"Sans, look at me," you sit up a little so you're eye to eye socket with him. "Do you actually think I would hurt your brother?"

"No," he laughs softly. "You two are the most sickeningly cute couple in existence. You could go for the Nose Nuzzle Championship if, you know, Paps had a nose."

"Exactly," you kiss his forehead again. "And I'm telling you the truth. Papyrus loves you, and he wants you to be happy...and so do I."

The words are bold, but you mean them. You watch his face for a minute, hoping you didn't just take one risk too many. Finally, he reaches up and hooks a fingertip under the collar of your shirt.

"You sounded like you were having a good time," he says softly, sliding his hand back and forth.

"We were," you reply, running your hand down the back of his ribcage. "Do you want me to show you what we did?"

Sans nods, and you shift up a little, until you're almost sitting upright. You slide his jacket off gently and take one of his hands, bringing it up to your mouth.

"Tell me what feels good," you say, repeating as best you can what you'd told Papyrus last night. "And tell me if anything feels bad, okay?"

"Okay," Sans says, and his voice is shaking even harder than his brother's had. You smile and press soft kisses to his knuckles, the narrow bones of his palm, and the bumps of his wrist. You can feel the magic that binds his body together, and it feels so similar to the magic you felt last night. For some reason, you hadn't expected it to feel this _strong_ , but the pulse of magic under your lips is warm and rich, tingling softly where it touches your skin. You take a fingertip into your mouth, then two, flicking your tongue against the ridges underneath. Sans gasps, eye sockets wide, and you smile, keeping them between your teeth.

Sans removes his hand from your mouth and pulls his shirt off, tossing it behind him with his jacket, and sure enough you can see his soul glowing bright blue inside his ribcage. He's panting now, and the glow from his soul spreads farther than Papyrus's did, all the way down to the inside of his pelvic bones. You lift both hands and gently run them down his sides, just barely touching, and he shivers.

"Huh. That's interesting. Papyrus was more sensitive here," you bring one hand up to his collarbone and run your thumb along its length. He hums softly, but doesn't shake. "And here..."

You gently wrap your fingertips around his spine, twisting them slowly, and Sans doubles forward and moans aloud. You bite your lip, thrilled to know the brothers had something in common in _that_ regard, and you bring your other hand up to lower the waistband of his shorts.

What you find gives you significant pause. It seems the glow in his pelvis hadn't been from his soul, after all.

"...oh _wow_ ," you say softly. "Yeah, Papyrus didn't have one of those."

"Heh. Yeah, it takes some practice." Sans says a little breathlessly. "I can actually make it look like a couple of different things, but right now..." 

His eye flashes, and it dissipates, leaving smooth bone underneath.

"I don't think it's necessary."

You smile and begin to move your hands again, one on his spine and one sliding over the crest of his hip. He shivers again, soft sounds turning to loud sounds as you lean forward and wrap your lips around one of his ribs.

"Holy shit," he gasps. "You're _really_ good at this." 

"Yeah?" you whisper against his bones, and when your breath touches his soul he grabs onto your shoulders. His hips move, pressing his leg more firmly between yours for a moment, and you groan involuntarily.

You feel a sudden slick warmth against the side of your face, and when you lean back, you see his soul pressed up against his ribs. You can take a hint, so you bring both hands up underneath his ribcage and stroke your fingertips along the underside. Shimmering blue magic drips down your fingertips, and you lave your tongue up his ribs, the sensation alternating between rough bone and wet magic. Sans clutches at your back, his breath coming faster and faster until he gives a choked gasp that turns into a shaky sigh. He trembles like a leaf in the wind, and under your touch his soul shines like a star.

After a moment, he collapses against you, and the magic of his soul disperses again into the rest of his body. You let yourself fall gently against the couch, furiously turned on but also weirdly exhausted. Papyrus's soul had been too sensitive to touch; just sliding your hands along his hips and spine had driven him over the edge. This is new...and you like it.

Eventually, Sans sits up a little and looks at you. You grin a little sheepishly. 

"That's...a little different than what Paps and I did," you admit. 

"I kinda figured," he chuckles, "and I'm willing to forgive you on one condition."

"Go on," your eyebrows lift.

"You show me what my brother did to you," he slides a hand under your shirt, "if you're up for it."

"God yes." 

Sans rolls off of you, and you wiggle your pants off so he can reach you. Given what Sans has just told you about his own anatomy, it's not a surprise that he needs less instruction than Papyrus did about the basics of yours. It's still just as thrilling to show him what you like, though, especially once he gets the hang of it.

"That feel good?" He asks. 

You just bite your lip and nod, quickly becoming overwhelmed by his movements.

"Felt good when my brother did it to you, didn't it?" Another nod. "And you made him feel good too, right? Just like you did to me?"

"Fuck yes," you hiss against Sans's collarbone. "God, he looked so beautiful. He just kept shaking and gasping and _ah-_ "

You come with a shout, your hips jerking upward as pleasure burns its way through you. When you fall back down, you can't seem to catch your breath for a long while, and you don't notice you've thrown your arm over your eyes until Sans moves it. He looks almost nervous, looking at the scattered clothing around you both hesitantly.

"Wow," he says softly. "That...sure did happen, didn't it?"

You laugh so hard, you don't hear the front door opening. The voice that follows, however, is impossible _not_ to hear.

"HUMAN!"

Despite everything that's just happened, you jump so hard you nearly fall straight off the couch. You feel your face heat up, feeling guilty for reasons you know you shouldn't.

"Hey, Paps," you say softly, glancing behind him to make sure he didn't bring any guests over. Thankfully, he didn't; the snow on his boots indicated that he'd just finished a long day of Royal Guard Duty. The flower petals stuck under the snow confirmed that suspicion.

"OH HUMAN, HOW WONDERFUL!" Papyrus bounds over and picks you up, squeezing you tight. "YOU DECIDED TO SHARE YOUR WONDERFUL KNOWLEDGE WITH SANS, JUST LIKE I ASKED YOU TO!"

Still naked from the waist down, his clothes feel incredibly cold from the snow outside. He sets you down after a moment when you start shivering particularly hard. He then decides to pick up Sans, who at this point has barely gotten his shorts back on and has one arm in his jacket.

"ISN'T THIS FANTASTIC, SANS?" He nuzzles their bony foreheads together. "AND TO THINK, UNDYNE TORE THOSE PAGES OUT OF MY DATING MANUAL 'FOR MY OWN GOOD'!"

"Uh...Papyrus," Sans says hesitantly, "you know this isn't something people normally do with their brothers...right?"

There's a long moment of pause after that, during which you begin to fear that this whole thing is about to come crashing down, and you'll have to make a run for the surface and never set foot underground again, because if you do, Undyne will _murder_ you-

"SANS," Papyrus says, his voice unusually somber. "DESPITE YOUR BEST EFFORTS TO SHIELD ME FROM THIS FACT, I AM WELL AWARE THAT WE HAVE NEVER BEEN 'NORMAL'."

Sans's grin is wider than yours, but not by much. You tug them down onto the sofa and end up in the world's boniest cuddle pile, unable to believe how lucky you all are.


End file.
